A complex theological term applied to the process of the substance of bread and wine becoming the true substance of the body and blood of Christ. Transubstantiation only occurs when a validly ordained priest, in communion with the Holy See (i.e.: Rome) licitly celebrates mass according to the dictates of same.

In Christianity, The miraculous effect whereby through the blessing and invocation of a priest the bread and wine of the Communion meal become in a mystical sense the true body and blood of Jesus Christ, while still under the guise of bread and wine.

The transformation of bread and wine to the body and blood of Jesus Christ. Done in the ritual of the mass by Christians. In short, illogical bullshit.

Johnny: "People have been eating Jesus for two thousand years?"
Father Smith: "That's right."
Johnny: "Wouldn't His body be used up by now?"
Father Smith: ...oh, look Johnny. You're parents are leaving. See you next week.